1. Field of the Invention
Embodiments of the invention generally relate to methods of doping substrates, such as semiconductor substrates.
2. Description of the Related Art
Junctions, such as p-n junctions, are commonly used in semiconductor devices such as diodes, transistors, solar cells, light-emitting diodes, and integrated circuits. One method of forming the junctions is doping, which introduces a dopant material into an intrinsic material. Typically, incorporation of boron, phosphorus, and arsenic or other dopants into a material, such as a semiconductor material, is performed by one of three ways.
In one manner, dopants such as boron, phosphorus, or arsenic may be introduced into a substrate by low temperature (e.g., less than 100 degrees Celsius) plasma ion implantation. The process is generally performed at low temperature due to the use of photoresists to restrict doping to desired areas of the substrate. The photoresists utilized during the plasma ion implantation process generally contain hydrocarbons, which degrade or “burn” at temperatures in excess of 150 degrees Celsius, making the photoresist difficult to remove after processing. In order to dope the substrate to the desired concentration, a significant substrate bias is employed to compensate for the reduced activity of the dopant species. However, the relatively large substrate bias, and the resulting ion acceleration energy (such as about 500 eV), creates interstitial defects in the substrate material. The interstitial defects promote dopant migration and diffusion (and therefore the depth of the dopant within the substrate, thus increasing the difficulty to form ultrashallow junctions (e.g., junctions have a depth of about 50 nanometers or less).
A second method of introducing dopants is beam line ion implantation. However, beam line ion implantation also utilizes ion implantation energies of about 500 eV, similarly resulting in interstitial defects which lead to excessive dopant migration and diffusion.
A third method of introducing dopants into a substrate is solid phase doping, in which dopants are incorporated into a substrate material via thermal migration. During a solid phase doping process, a substrate is heated to a surface temperature of about 1000 degrees Celsius or greater and exposed to a dopant, for example a dopant-containing gas. Due to the relatively high temperatures (e.g., greater than 1000 degrees Celsius), dopants diffuse form the dopant-containing gas into the substrate material. However, due to the relatively high temperatures needed to cause sufficient dopant diffusion into the substrate, and the diffusion coefficient at these high temperatures, the dopants diffuse significantly into the wafer. The significant diffusion of the dopants often results in the dopants diffusing excessively beyond the desired depth, thus increasing the difficulty to form ultrashallow junctions.
Therefore, there is a need for an improved method of forming ultrashallow junctions via doping.